Various cellular radio systems or mobile telephone systems are in use and presently being designed in which the geographical coverage area of the system is divided into smaller separate radio areas, i.e. cells, in such a way that when the radiotelephone or mobile telephone is located in a cell, it communicates with the fixed network through a fixed radio station incorporated in the cell. The mobile telephones belonging to the system can roam freely within the system area from one cell to another. One such system is the digital mobile telephone system GSM (Global System for Mobiles).
Cordless telephone (CT) systems, such as the DECT system, have recently come out on the market. The DECT system comprises a base station, which is normally based on access to an existing fixed telephone network. There are three main types of base stations: a residential base station, a base station connected to a private branch exchange in an office, and a telepoint base station.
Cordless telephone systems can also be connected to mobile telephone networks, for instance to the GSM system. The private branch exchange (PBX) of a cordless telephone system can be connected to the GSM system for example by using DSS 1, Q.930-Q.940, 1989, Digital Subscriber Signalling System No. 1, Network Layer, User-Network Management, CCITT signalling supplemented with mobility management messages. The PBX of a cordless telephone system is connected to the switching centre of the GSM system so that the subscribers in the PBX can roam within the service area of the GSM system, and the subscribers in the PBX can make use of the mobility management procedures of the GSM system and roam within the service area of DECT systems connected to the GSM system. In such a case, mobile subscribers are attached to the PBX in accordance with the DECT radio path protocol ETS 300175, Radio Equipment and Systems (RES), Digital European Cordless Telecommunications (DECT), Common Interface, for instance. It has also been possible to connect fixed subscribers to the PBX, for example in accordance with the ISDN signalling protocol. In that event, calls originating from the GSM system or from a GSM-connected telephone network can also be routed to cordless, for instance DECT, subscribers located within the service area of the PBX.
In the prior art solution, when there is an inbound call to the PBX, register interrogation procedures in accordance with the GSM recommendations are performed, irrespective of the location of the called subscriber. FIG. 1a shows call establishment procedures in accordance with the prior art. When the call arrives at a first mobile-services switching centre, which may be a gateway mobile services switching centre (GMSC), the GMSC requests 101 routing information from the home location register (HLR) of the called subscriber by sending a SEND.sub.-- ROUT.sub.-- INFO message. The home location register HLR checks the subscriber data from its subscriber database and sends 102 a roaming number request PROV.sub.-- ROAM.sub.-- NUM to the visitor location register VLR of the location area of the subscriber, and the VLR sends 103 a roaming number and routing information as an acknowledgement ROAM.sub.-- NUM-ACK to the home location register HLR. The home location register HLR transmits 104 the routing information and roaming number as an acknowledgement ROUT.sub.-- INFO.sub.-- ACK to the gateway mobile services switching centre (GMSC), which sends 105 an indication of the subscriber, the inbound call and the roaming number in an initial address message (IAM) (specified in the common channel signalling standard Q.721-Q.766, 1989, Specifications of Signalling System No. 7, CCITT, Vol. VI--Fascicle VI.8, p. 251) to the mobile services switching centre MSC of the location area of the subscriber. Having received the IAM message, the mobile services switching centre MSC requests 106 the visitor location register VLR of the location area of the subscriber to provide routing information for the call to be routed by way of a S.sub.-- INFO.sub.-- F.sub.-- I/C.sub.-- CALL message. After having received the message or request, the visitor location register VLR performs a subscriber data check and initiates the prior art mobility management functions which form part of the normal operations for establishing a connection and are set forth in greater detail in the GSM recommendation GSM 09.02, Mobile Application Part Specification, ETSI. The first mobility management function is the paging function 107, by way of which the subscriber is located prior to call establishment. After the paging, the mobile services switching centre MSC sends 108 a resource request PROCESS.sub.-- ACC.sub.-- REQ to the visitor location register VLR. This is followed by authentication 109 of the subscriber. Authentication is a process wherewith the subscriber is identified as a legal user of the system. Subsequent to the authentication, ciphering 110 of the messages employed is performed. Thereafter the visitor location register VLR responds to the resource request by sending 111 an acknowledgement ACCESS.sub.-- REQ.sub.-- ACC. Following this, assignment 112 of the temporary mobile subscriber identity of the mobile subscriber, TMSI.sub.-- ASSIGNMENT, is performed. The procedure according to this Figure is continued in FIG. 1b.
FIG. 1b is a continuation of the prior art call establishment shown in FIG. 1. When all the connection establishment functions 106, 107, 108, 109, 110, 111 and 112 shown in FIG. 1a have been performed, the visitor location register VLR sends 113 a COMPLETE.sub.-- CALL message by which the visitor location register indicates that a call can be established. Only thereafter is the mobile services switching centre MSC free to establish a call to the subscriber. In that situation, the mobile services switching centre MSC sends a connection establishment request 114 as a SETUP message to the home private branch exchange PBX of the subscriber. After that, the PBX sends a connection establishment CC.sub.-- SETUP message 119 to the subscriber station CPP (Cordless Portable Part), that is, to the hand-held mobile telephone attached to the PBX. It is to be noted that fixed subscribers may also access to the PBX. The subscriber station CPP responds to the CC.sub.-- SETUP message by sending 120 a CC.sub.-- ALERTING message to the PBX. Following this, the PBX sends 121 a corresponding ALERT message to the mobile services switching centre MSC, which sends 122 an address complete message (ACM) further to the gateway mobile services switching centre GMSC, indicating that all digits required for routing the call have been received. When a connection has been established between the subscriber station CPP and the PBX, the subscriber station CPP sends 123 a CC.sub.-- CONNECT message to the PBX, and the PBX transmits 124 this message to the mobile services switching centre MSC as a CONNECT message. The PBX acknowledges the message sent to the mobile services switching centre by sending 125 a CC.sub.-- CONNECT.sub.-- ACK message to the subscriber station CPP. Respectively, the mobile services switching centre acknowledges the received CONNECT message by sending 126 a CONNECT.sub.-- ACK acknowledgement message to the home private branch exchange PBX of the subscriber. Thereafter, the mobile services switching centre indicates that the subscriber has responded to the call by sending 127 a response message ANM to the gateway mobile services switching centre GMSC.
It is apparent from the foregoing that establishment of a connection to a subscriber located within the service area of a private branch exchange connected to a cellular radio network is a complicated and cumbersome process. It should be noted that the prior art call establishment procedure as set forth above must always be performed when a subscriber located within the service area of the private branch exchange is to be called. The mobility management functions to be performed in the GSM system which form part of the operations for connection establishment, i.e. the paging 107, subscriber authentication 109, ciphering 110, TMSI assignment 112, the S.sub.-- INFO.sub.-- F.sub.-- I/C.sub.-- CALL 106, PROCESS.sub.-- ACC.sub.-- REQ 108, ACCESS.sub.-- REQ.sub.-- ACC 111 and COMPLETE.sub.-- CALL 113 messages to be sent, the roaming number request PROV.sub.-- ROAM.sub.-- NUM 102 placed by the home location register HLR and the response ROAM.sub.-- NUM.sub.-- ACK 103 provided by the visitor location register VLR, as described in the GSM 09.02 recommendation, considerably load the signalling capacity of the mobile telephone network. Heavy loading of the signalling capacity may lead to an overload and system failure, and naturally gives rise to unnecessary traffic in the network. It is further to be noted that the above connection establishment procedures and sending of the outbound messages mentioned above load the network elements that transmit such messages. The processor capacity of the network elements is unnecessarily occupied and may be overloaded. Furthermore, the network elements receiving the messages must process them and perform the desired functions which again load further network elements that receive messages.